Reduced size force sensor

ABSTRACT

A flexible probe has an assembly in its distal end that includes a transmitter and a receiver that receives signals from the transmitter for sensing a position of the receiver relative to the transmitter. A resilient element disposed between the transmitter and the receiver is configured to deform in response to pressure exerted on the distal tip when the distal tip engages a wall of a body cavity.

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BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention relates to instruments for diagnostic and surgicalpurposes. More particularly, this invention relates to measurements offorce, pressure or mechanical tension or compression using catheters fordiagnostic and surgical procedures in the heart.

2. Description of the Related Art

Cardiac arrhythmias, such as atrial fibrillation, occur when regions ofcardiac tissue abnormally conduct electric signals to adjacent tissue,thereby disrupting the normal cardiac cycle and causing asynchronousrhythm.

Procedures for treating arrhythmia include surgically disrupting theorigin of the signals causing the arrhythmia, as well as disrupting theconducting pathway for such signals. By selectively ablating cardiactissue by application of energy via a catheter, it is sometimes possibleto block or modify the propagation of unwanted electrical signals fromone portion of the heart to another. The ablation process destroys theunwanted electrical pathways by formation of non-conducting lesions.

Verification of physical electrode contact with the target tissue isimportant for controlling the delivery of ablation energy. Attempts inthe art to verify electrode contact with the tissue have been extensive,and various techniques have been suggested. For example, U.S. Pat. No.6,695,808 describes apparatus for treating a selected patient tissue ororgan region. A probe has a contact surface that may be urged againstthe region, thereby creating contact pressure. A pressure transducermeasures the contact pressure. This arrangement is said to meet theneeds of procedures in which a medical instrument must be placed in firmbut not excessive contact with an anatomical surface, by providinginformation to the user of the instrument that is indicative of theexistence and magnitude of the contact force.

As another example, U.S. U.S. Pat. No. 6,241,724 describes methods forcreating lesions in body tissue using segmented electrode assemblies. Inone embodiment, an electrode assembly on a catheter carries pressuretransducers, which sense contact with tissue and convey signals to apressure contact module. The module identifies the electrode elementsthat are associated with the pressure transducer signals and directs anenergy generator to convey RF energy to these elements, and not to otherelements that are in contact only with blood.

A further example is presented in U.S. Pat. No. 6,915,149. This patentdescribes a method for mapping a heart using a catheter having a tipelectrode for measuring local electrical activity. In order to avoidartifacts that may arise from poor tip contact with the tissue, thecontact pressure between the tip and the tissue is measured using apressure sensor to ensure stable contact.

U.S. Patent Application Publication 2007/0100332 describes systems andmethods for assessing electrode-tissue contact for tissue ablation. Anelectromechanical sensor within the catheter shaft generates electricalsignals corresponding to the amount of movement of the electrode withina distal portion of the catheter shaft. An output device receives theelectrical signals for assessing a level of contact between theelectrode and a tissue.

Commonly assigned U.S. Patent Application Publication No. 2009/0093806to Govari et al., which is herein incorporated by reference, describesanother application of contact pressure measurement, in whichdeformation in response to pressure on a resilient member located at thedistal end of a catheter is measured using a sensor.

SUMMARY OF THE INVENTION

There is provided according to embodiments of the invention a flexibleprobe that is adapted for insertion into a body cavity of a livingsubject. The probe has an assembly in its distal end that includes atransmitter and a receiver that receives signals from the transmitterfor sensing a position of the receiver relative to the transmitter. Aresilient element disposed between the transmitter and the receiver isconfigured to deform in response to pressure exerted on the distal tipwhen the distal tip engages a wall of the body cavity.

According to an aspect of the apparatus, the transmitter includes asingle coil. There may be a high permeability core disposed in the coil.

According to another aspect of the apparatus, the receiver has threecoils. There may be respective high permeability cores disposed in thethree coils.

According to a further aspect of the apparatus, the assembly alsoincludes a plurality of localizer coils that are operative to respond toincident radio-frequency radiation from an external source, thelocalizer coils is integrated as part of a receiver coil circuitry.

According to another aspect of the apparatus, the resilient element is anitinol spring.

According to still another aspect of the apparatus, the resilientelement is a tubular segment of an elastic material having a pluralityof helical cuts formed therethrough.

According to a further aspect of the apparatus, the resilient element isa single coil spring, which can be a nitinol spring.

According to one aspect of the apparatus, the single coil spring hasgaps occupied by a permeable material.

According to an additional aspect of the apparatus, the assemblyincludes a heat-resistant flexible plastic sheath that covers theresilient element.

There is further provided according to embodiments of the invention amethod, which is carried out by providing a flexible probe that isadapted for insertion into a body cavity of a living subject. Anassembly in the distal end of the probe includes a transmitter, areceiver and a resilient element disposed between the transmitter andthe receiver. The method is further carried out by deforming theresilient element by exerting pressure on the distal tip when the distaltip engages a wall of the body cavity, and while deforming the resilientelement emitting signals from the transmitter, receiving the signals inthe receiver and processing the signals to determine a position of thereceiver relative to the transmitter.

According to an aspect of the method, processing the signals includescalculating the position of the receiver relative to the transmitterresponsively to an amplitude of the signals.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a better understanding of the present invention, reference is madeto the detailed description of the invention, by way of example, whichis to be read in conjunction with the following drawings, wherein likeelements are given like reference numerals, and wherein:

FIG. 1 is a pictorial illustration of a system for evaluating electricalactivity in a heart of a living subject in accordance with an embodimentof the invention;

FIG. 2 is a partially cut-away view of distal portion of a catheter inaccordance with an embodiment of the invention;

FIG. 3 is a subassembly suitable for use in the catheter shown in FIG. 2in accordance with an embodiment of the invention;

FIG. 4 is an elevation of the distal portion of a cardiac catheter inaccordance with an embodiment of the invention;

FIG. 5 is a magnified elevation of an assembly in the distal portion ofa cardiac catheter in accordance with an alternate embodiment of theinvention;

FIG. 6 is a schematic sectional view of the distal end of a catheter inaccordance with an alternate embodiment of the invention;

FIG. 7 is an exploded view of the assembly shown in FIG. 6 in slightperspective in accordance with an embodiment of the invention;

FIG. 8, which is top view of a planar assembly of a contact force sensorwith integrated location coils in accordance with an embodiment of theinvention;

FIG. 9, which is an oblique view of a spring assembly in accordance withan embodiment of the invention; and

FIG. 10 is a side elevation of the assembly shown in in FIG. 9 inaccordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the various principles ofthe present invention. It will be apparent to one skilled in the art,however, that not all these details are necessarily needed forpracticing the present invention. In this instance, well-known circuits,control logic, and the details of computer program instructions forconventional algorithms and processes have not been shown in detail inorder not to obscure the general concepts unnecessarily.

Documents incorporated by reference herein are to be considered anintegral part of the application except that, to the extent that anyterms are defined in these incorporated documents in a manner thatconflicts with definitions made explicitly or implicitly in the presentspecification, only the definitions in the present specification shouldbe considered.

System Overview

Turning now to the drawings, reference is initially made to FIG. 1,which is a pictorial illustration of a system 10 for evaluatingelectrical activity and performing ablative procedures on a heart 12 ofa living subject, which is constructed and operative in accordance witha disclosed embodiment of the invention. The system comprises a catheter14, which is percutaneously inserted by an operator 16 through thepatient's vascular system into a chamber or vascular structure of theheart 12. The operator 16, who is typically a physician, brings thecatheter's distal tip 18 into contact with the heart wall, for example,at an ablation target site. Electrical activation maps may be prepared,according to the methods disclosed in U.S. Pat. Nos. 6,226,542, and6,301,496, and in commonly assigned U.S. Pat. No. 6,892,091, whosedisclosures are herein incorporated by reference. One commercial productembodying elements of the system 10 is available as the CARTO® 3 System,available from Biosense Webster, Inc., 3333 Diamond Canyon Road, DiamondBar, Calif. 91765. This system may be modified by those skilled in theart to embody the principles of the invention described herein.

Areas determined to be abnormal, for example by evaluation of theelectrical activation maps, can be ablated by application of thermalenergy, e.g., by passage of radiofrequency electrical current throughwires in the catheter to one or more electrodes at the distal tip 18,which apply the radiofrequency energy to the myocardium. The energy isabsorbed in the tissue, heating it to a point (typically above 50° C.)at which it permanently loses its electrical excitability. Whensuccessful, this procedure creates non-conducting lesions in the cardiactissue, which disrupt the abnormal electrical pathway causing thearrhythmia. The principles of the invention can be applied to differentheart chambers to diagnose and treat many different cardiac arrhythmias.

The catheter 14 typically comprises a handle 20, having suitablecontrols on the handle to enable the operator 16 to steer, position andorient the distal end of the catheter as desired for the ablation. Toaid the operator 16, the distal portion of the catheter 14 containsposition sensors (not shown) that provide signals to a processor 22,located in a console 24. The processor 22 may fulfill several processingfunctions as described below.

Ablation energy and electrical signals can be conveyed to and from theheart 12 through one or more ablation electrodes 32 located at or nearthe distal tip 18 via cable 34 to the console 24. Pacing signals andother control signals may be conveyed from the console 24 through thecable 34 and the electrodes 32 to the heart 12. Sensing electrodes 33,also connected to the console 24 are disposed between the ablationelectrodes 32 and have connections to the cable 34.

Wire connections 35 link the console 24 with body surface electrodes 30and other components of a positioning sub-system for measuring locationand orientation coordinates of the catheter 14. The processor 22 oranother processor (not shown) may be an element of the positioningsubsystem. The electrodes 32 and the body surface electrodes 30 may beused to measure tissue impedance at the ablation site as taught in U.S.Pat. No. 7,536,218, issued to Govari et al., which is hereinincorporated by reference. A temperature sensor (not shown), typically athermocouple or thermistor, may be mounted on or near each of theelectrodes 32.

The console 24 typically contains one or more ablation power generators25. The catheter 14 may be adapted to conduct ablative energy to theheart using any known ablation technique, e.g., radiofrequency energy,ultrasound energy, cryogenic energy, and laser-produced light energy.Such methods are disclosed in commonly assigned U.S. Pat. Nos.6,814,733, 6,997,924, and 7,156,816, which are herein incorporated byreference.

In one embodiment, the positioning subsystem comprises a magneticposition tracking arrangement that determines the position andorientation of the catheter 14 by generating magnetic fields in apredefined working volume and sensing these fields at the catheter,using field generating coils 28. The positioning subsystem is describedin U.S. Pat. No. 7,756,576, which is hereby incorporated by reference,and in the above-noted U.S. Pat. No. 7,536,218.

As noted above, the catheter 14 is coupled to the console 24, whichenables the operator 16 to observe and regulate the functions of thecatheter 14. Console 24 includes a processor, preferably a computer withappropriate signal processing circuits. The processor is coupled todrive a monitor 29. The signal processing circuits typically receive,amplify, filter and digitize signals from the catheter 14, includingsignals generated by sensors such as electrical, temperature and contactforce sensors, and a plurality of location sensing electrodes (notshown) located distally in the catheter 14. The digitized signals arereceived and used by the console 24 and the positioning system tocompute the position and orientation of the catheter 14, and to analyzethe electrical signals from the electrodes.

In order to generate electroanatomic maps, the processor 22 typicallycomprises an electroanatomic map generator, an image registrationprogram, an image or data analysis program and a graphical userinterface configured to present graphical information on the monitor 29.

Typically, the system 10 includes other elements, which are not shown inthe figures for the sake of simplicity. For example, the system 10 mayinclude an electrocardiogram (ECG) monitor, coupled to receive signalsfrom one or more body surface electrodes, in order to provide an ECGsynchronization signal to the console 24. As mentioned above, the system10 typically also includes a reference position sensor, either on anexternally applied reference patch attached to the exterior of thesubject's body, or on an internally placed catheter, which is insertedinto the heart 12 maintained in a fixed position relative to the heart12. Conventional pumps and lines for circulating liquids through thecatheter 14 for cooling the ablation site are provided. The system 10may receive image data from an external imaging modality, such as an MRIunit or the like and includes image processors that can be incorporatedin or invoked by the processor 22 for generating and displaying images.

First Embodiment

Reference is now made to FIG. 2 and to FIG. 3, which are respectively apartially cut-away view of distal portion 41 of a catheter and aschematic, partially exploded view an assembly 109 in the distal portion41 in accordance with embodiments of the invention. As shown in FIG. 2,the distal portion 41 has an ablation electrode 43. A temperature sensor57 may be present in the distal portion 41 to monitor temperatures atthe ablation site. A flexible spring, helix 63, is a tubular piece of anelastic material having a plurality of intertwined helical cutstherethrough along a portion of a length of the piece, which contractsand expands along axis of symmetry 51 as the contact force between thecatheter and tissue varies.

Contact force sensor 53, which includes the helix 63, is disposed in thedistal portion proximal to the ablation electrode 43. The contact forcesensor 53 comprises by a radiofrequency receiver—transmitter combination(not shown in FIG. 2). In this embodiment the receiver is proximal tothe transmitter. However, they may be disposed in the opposite order.The contact force sensor 53 forms a deformable coupling member withinthe distal portion 41. The two part implementation simplifies assemblyof a magnetic field generator and magnetic position sensor into themember.

The assembly 109 is typically covered by a flexible plastic sheath 87.When catheter 69 is used, for example, in ablating endocardial tissue bydelivering radio-frequency electrical energy through electrode 89,considerable heat is generated in the area of distal tip 49. For thisreason, it is desirable that plastic sheath 87 comprises aheat-resistant plastic material, such as polyurethane, whose shape andelasticity are not substantially affected by exposure to the heat. Mostimportantly, plastic sheath 87 serves to keep blood out of the interiorof the catheter.

As best appreciated in FIG. 3, the contact force sensor 53 comprises apaired radiofrequency transmitter and receiver. The receiver is a set ofthree coils 94, optionally provided with internal ferrite cores 111 forsignal enhancement. The coils 94 face a transmitting coil 113, which isa single frequency loop antenna that emits radiofrequency signals thatare received in the coils 94. The three coils 94 generate signals fromthe incident radiofrequency radiation produced by transmitting coil 113.The amplitude of the received radiofrequency signals varies generallyinversely with the distance between the coils 94 and the transmittingcoil 113, and thus provides a measure of the contact force-dependentdeformation of the helix 63. As illustrated in FIG. 3, a single coil 63Aresilient member is disposed between the transmitting coil 113 and thereceiver coils 94. As will be seen from the description of theembodiments below, the transmitter and the receiver can be implementedrespectively as planar printed circuit boards (PCBs). This reduces theoverall size of the contact force sensor 53.

The assembly 109 comprises localizer coils 115 that function as alocation detector by generating position-dependent signals from incidentRF radiation produced by external field generating coils 28 (FIG. 1).The field generating coils 28 (typically nine) are fixed in a locationpad that is positioned beneath a patient. The localizer coils 115 arecircumscribed by the three coils 94.

In some embodiments the signals received in the three coils 94 may bedistinguished by using different frequencies in the transmitting coil113. Analysis of the force-dependent signals gives the magnitude of theforce on the distal tip. The analysis may also reveal the orientation ofthe distal tip with respect to the axis of the proximal end of the helix63, i.e., the amount of bending of the helix 63 about axis of symmetry51.

A fuller description of a force sensor using these components is givenin PCT Patent Document WO96/05768 of Ben Haim, commonly assigned U.S.Patent Application Publications No. 2011/0130648 and 2009/0093806 andcommonly assigned application Ser. No. 14/974,731, which are hereinincorporated by reference.

Reference is now made to FIG. 4, which is an elevation of the distalportion of a cardiac catheter 117 in accordance with an embodiment ofthe invention. The catheter 117 has an ablation electrode 119 at itsdistal end, and a resilient contact force sensor assembly 121 thatincludes a contact force sensor. Visible are a plastic sheath 123 thatextends to the proximal portion of the ablation electrode 119. Helicalspring 125 is formed as a cut-out in tubular plastic material 127. Alocalizer coil 129 is disposed proximal to the spring 125. Thetransmitter and receiver shown in FIG. 3 are present, but not seen inFIG. 4.

Second Embodiment

Reference is now made to FIG. 5, which is a magnified view of anassembly 131 in the distal portion of a cardiac catheter in accordancewith an alternate embodiment of the invention. The assembly 131 issimilar to the assembly 121 (FIG. 3), except now a nitinol spring 133 isemployed in the contact force sensor. Plastic sheath 135 covers thespring 133. The spring 133 slides with respect to the sheath 135. Theinner diameter of the sheath 135 is larger than the outer diameter ofthe spring 133. Edges of the receiving coils 137 and transmitting coils141 are seen beneath the spring 133. A magnetically permeable material139 resides on top of both transmitting and receiving coils.

Third Embodiment

Reference is now made to FIG. 6, which is a schematic sectional view ofthe distal end of a catheter 143 in accordance with an alternateembodiment of the invention. In this embodiment an assembly 145 containstwo flat spring coils 147. Typically the assembly 145 has a diameter of2.5 mm and a length of 1 mm. Transmitter 91 and receiving coils 94 ofreceiver 93 are disposed on opposite sides of the assembly 145, and maycomprise printed circuit boards. Conductors 95, 97 supply thetransmitter 91 and receiver 93.

Reference is now made to FIG. 7, which is an exploded view of theassembly 145 (FIG. 5) shown in slight perspective in accordance with anembodiment of the invention. The assembly 145 has a transmitter retainer149 and a receiver retainer 151. The transmitter 91 and receiver 93 (notshown in FIG. 6) may be attached to these retainers. The transmitterretainer 149 and receiver retainer 151 respectively mate with externaledges of a first flat spring coil 153 and a second flat spring coil 155.The flat spring coils 153, 155 are held apart by a spacer 157, whichmates with internal edges of the flat spring coils 153, 155. The flatspring coils 153, 155 deform in response to a compressive force thaturge the transmitter retainer 149 and the receiver retainer 151 towardone another as indicated by arrows 159 in FIG. 6. The flat spring coils153, 155 return to a resting state when the compressive force isremoved.

The flat spring coils 153, 155 can be mass produced to reduce unit cost.The designs can be cut, stamped, or otherwise formed from planar sheetmetal such as flat nitinol sheet and may be shape-set into their finalforms. Minimizing thickness of the elastic portion of the spring isimportant in cardiac catheters, as the transmitter retainer 149 andreceiver retainer 151 are at opposite ends of the contact force sensor,separating the transmitter 91 and receiver 93. (FIG. 5). The transmitter91 and receiver 93 are separated by a distance in the range of 0.1-1.5mm. Moreover, by laser-cutting the sheet metal into a pattern, no weldsare necessary, which keeps unit cost low, as well as improvingreliability relative to conventional welded springs.

Further details of techniques for manufacturing spring coils that aresuitable, mutatis mutandis, for the flat spring coils 153, 155 aredisclosed in commonly assigned, copending application Ser. No.15/347,242, entitled “Coils Formed in Folded Nitinol Sheet”, whosedisclosure is herein incorporated by reference.

As in the previous embodiment, the assembly 145, the transmitter 91 andthe receiver 93 may be constructed as an integral module with anelectrical connection between the transmit and receive section.

Fourth Embodiment

In this embodiment, the transmitter and receiver are planar structuresattached to opposite ends of a flat spring coil. The distance betweenthe transmitter and receiver varies as the spring coil deforms andrelaxes. Reference is now made to FIG. 8, which is top view of a planarassembly 161 in a contact force sensor with integrated location coils inaccordance with an embodiment of the invention. The assembly 161 can bemounted at either end of a flat spring coil (not shown), and haselectronic circuitry 163 arranged formed as a circuit board, andconfigured as a transmitter or a receiver. The circuit board can becovered with a material having high magnetic permeability in order toimprove magnetic alignment. The material can be mu-metal, for example,in the form of trapezoids that conform to the shape of the circuitboard. The electronic circuitry 163 that is connected to three coils 165circularly arranged at 120 degree angles. In this example the coils 165are used as receiving coils.

The arrangement for transmitting coils is similar. When the coils 165are used as transmitting coils, the transmitter comprises threeindividual transmitters. From the description below, it will be seenthat the transmitting coils align with respective receiving coils, whichincreases the accuracy of the readings of the contact force sensor. Thethree transmitting coils may be connected so that they are either inseries and can be powered with one AC generator or are in parallel wherethey can be run at different frequencies by different AC generators.

Also shown are optional windings 167. The windings 167 are components ofthe positioning sub-system noted in FIG. 1, which is outside the scopeof this disclosure.

Reference is now made to FIG. 9, which is an oblique view of a springassembly 169 in accordance with an embodiment of the invention. Threeplanar transmitting coils 171 oppose receiving coils 173 on oppositeends of a compressible spring 175 arranged as in a helix having at leastthree windings and flat surfaces.

Reference is now made to FIG. 10, which is a side elevation of theassembly 169 in accordance with an embodiment of the invention. Thetransmitting coils 171 and receiving coils 173 are flexible, and remainapplied to the upper and lower flat surfaces 177, 179 of the spring 175as the spring deforms responsively to compressive force acting againstupper and lower legs 181, 183. The terms “upper” and “lower” are usedarbitrarily herein to distinguish opposite directions. These terms haveno physical meanings with respect to the actual configuration of theassembly 169.

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed hereinabove. Rather, the scope of the present inventionincludes both combinations and sub-combinations of the various featuresdescribed hereinabove, as well as variations and modifications thereofthat are not in the prior art, which would occur to persons skilled inthe art upon reading the foregoing description.

The invention claimed is:
 1. A reduced size contact force sensorapparatus, comprising: a flexible probe having a proximal portion and adistal end, the probe adapted for insertion into a body cavity of aliving subject, the probe having a distal tip comprising an electrode atthe distal end of the probe; and an assembly in the distal end of theprobe, comprising: a flexible spring disposed in the distal end proximalto the electrode, the flexible spring having a plurality of intertwinedhelical cuts therethrough along a portion of a length of the spring, theflexible spring being configured to deform in response to pressureexerted on the distal tip when the distal tip engages a wall of the bodycavity; a transmitter disposed within the flexible spring; a receiverdisposed within the flexible spring configured to receive signals fromthe transmitter for sensing a position of the receiver relative to thetransmitter; and a single coil spring disposed within the flexiblespring between the transmitter and the receiver, the single coil springbeing configured to deform commensurate with the flexible spring inresponse to pressure exerted on the distal tip when the distal tipengages a wall of the body cavity and reduce the distance between thetransmitter and the receiver as the flexible spring and single coilspring deform.
 2. The apparatus according to claim 1, wherein thetransmitter comprises a single coil.
 3. The apparatus according to claim2, wherein the transmitter further comprises a high permeability coredisposed in the coil.
 4. The apparatus according to claim 1, wherein thereceiver comprises three coils.
 5. The apparatus according to claim 4,wherein the receiver further comprises respective high permeabilitycores disposed in the three coils.
 6. The apparatus according to claim4, wherein the assembly further comprises a plurality of localizer coilsthat are operative to respond to incident radiofrequency radiation froman external source, the localizer coils being integrated as part of areceiver coil circuitry.
 7. The apparatus according to claim 1, whereinthe assembly further comprises a heat-resistant flexible plastic sheath.8. A method comprising the steps of: providing a flexible probe having aproximal portion and a distal end, the probe adapted for insertion intoa body cavity of a living subject, the probe having a distal tipcomprising an electrode at the distal end of the probe; providing anassembly in the distal end of the probe, comprising: a flexible springdisposed in the distal end proximal to the electrode, the flexiblespring having a plurality of intertwined helical cuts therethrough alonga portion of a length of the spring, a transmitter disposed within theflexible spring, a receiver disposed within the flexible springconfigured to receive signals from the transmitter for sensing aposition of the receiver relative to the transmitter, and a single coilspring disposed within the flexible spring between the transmitter andthe receiver, the single coil spring being configured to deformcommensurate with the flexible spring in response to pressure exerted onthe distal tip when the distal tip engages a wall of the body cavity andreduce the distance between the transmitter and the receiver as theflexible spring and single coil spring deform, and; deforming theflexible spring by exerting pressure on the distal tip when the distaltip engages a wall of the body cavity; during the deforming of theflexible spring: emitting signals from the transmitter; receiving thesignals in the receiver and processing the signals to determine aposition of the receiver relative to the transmitter.
 9. The methodaccording to claim 8, wherein processing the signals comprisescalculating the position of the receiver relative to the transmitterresponsively to an amplitude of the signals.
 10. The method according toclaim 8, further comprising covering the assembly with a heat-resistantflexible plastic sheath.